Polyolefin resin microporous membranes have been used as separators for batteries, in particular, separators for lithium ion batteries. Lithium ion batteries have been used in small size electronic devices such as cellular phones and notebook-size personal computers, and also have been attempted to be applied to electric tools, hybrid vehicles, electric vehicles and the like.
For separators for lithium ion batteries, polyethylene microporous membranes have hitherto been used. This is because polyethylene microporous membranes are excellent in permeability, and have a function to perform shutdown of the current by blocking continuous pores through melting the polymer at 130° C. to 150° C., for the purpose of ensuring the safety of the batteries. The term “shutdown” means a phenomenon in which the pores of a microporous membrane are blocked by a molten resin to increase the electrical resistance of the membrane and consequently the membrane shuts down the flow of the lithium ions.
In this connection, from the viewpoint of more improving the safety of electricity storage devices, the separator is required to have, in addition to mechanical properties, above a certain level, not to be broken during repeated charge-discharge cycles, the properties such as the property (fuse property) to rapidly halt the battery reaction when abnormal heating occurs, and the property (short-circuit property) to prevent a dangerous situation of the direct reaction between the positive electrode material and the negative electrode material through maintaining the shape of the separator even when the temperature comes to be high.
It is recognized that the lower is the temperature at which the fuse occurs, the higher is the effect to safety. The higher temperature at which short-circuit occurs is preferable from the viewpoint of maintaining the film shape even after the blocking of the pores and maintaining the insulation between the electrodes.
Recently, for the purpose of further improving the safety of batteries, there have been proposed a method in which a layer mainly composed of an insulating inorganic filler is formed between the separator and each of the electrodes (for example, Patent Literature 1), and a separator made of a polyethylene microporous membrane including an inorganic substance (for example, Patent Literature 2 and Patent Literature 3).
As an attempt to increase the heat resistance of the separator, there have been performed an attempt to blend polypropylene high in melting point with polyethylene, and an attempt to laminate a polyethylene microporous membrane and a polypropylene microporous membrane (see, for example, Patent Literature 4 and Patent Literature 5).